Osteogenesis imperfecta (OI) is a genetic disorder in which the bones are extremely brittle and highly susceptible to fracture. Most cases of OI are caused by mutations in type I collagen genes that result in reduced amounts of normal collagen or structural defects of the triple helix, leading to abnormal fibril formation and/or assembly. Th disease spectrum in OI varies from severe forms with intrauterine fractures/perinatal lethality to mild forms without fractures. The current standard of care for OI is bisphosphonate treatment. However, recent concerns over the potential of these drugs to inhibit bone remodeling and impair fracture healing, as well as the lack of knowledge about the long term consequences of bisphosphonate treatment in children bring new urgency to the search for alternative OI therapies. Our laboratory has recently generated a green collagen mouse in which the collagen pro2(I) chain is fused with green fluorescent protein (GFP) and is expressed under control of the 3.6kb COL1A1 promoter for expression in osteoblasts in bone. Our preliminary data validating this novel 2(I)-GFP-collagen fusion protein have shown that it behaves similarly to the wild type form, is secreted upon addition of ascorbic acid, co-precipitates with collagen 1 chains, and is assembled into banded collagen fibril arrays. Mice expressing the GFP-collagen construct show green fluorescent collagen in the bone matrix, tendon, intervertebral discs and skin and appear phenotypically normal. These mice provide a novel and powerful new research tool with which to explore the utility of transplantation of whole marrow, stem cells or induced pluripotent stem cells (iPS) as potential therapies for repair of abnormal collagen in OI. In this exploratory R21, we plan to perform combined bone marrow transplantation of GFPcollagen expressing cells and osteoprogenitors into two mouse models of OI representing moderately severe and mild forms of OI. These include the oim mouse, which does not produce functional pro2(I) collagen chains and the G610C mouse, which carries a cysteine mutation in the 2(I) chain. The GFP-collagen expressing transplanted cells provide a powerful in vivo readout with which we can assess not only engraftment of donor cells, but also the degree of replacement of mutant (host) bone collagen with donor (GFP-positive) collagen. These innovative studies will determine the extent to which abnormal collagen can be replaced by the collagen from the transplanted cells as a function of time and explore potential approaches to enhance the extent and amount of collagen replacement through treatment with bone anabolic and antiresorptive agents. These studies have the potential to lead to novel or improved treatments for patients with OI.